There are probably a bunch in the archives on flats,but I'll give a couple thoughts on how we cook them.
Put a heavy rub of choice on it and let set a few hrs.
Put on a center rack,away from your firebox,set cooker between 200�-225�.
Use about four ounces of anything other than mesquite.
We just like some pecan.
Cook to 160�-165� and remove.
Ordinarily we don't foil much in a Cookshack,but learning flats it isn't a bad plan.
Add +/- 1/2 cup of low salt beef stock,maybe a dash of wooster,and a couple pats of butter to some foil wrap.
Put your temp probe back in and seal well.
Check it for tenderness when it gets over 185� by probing it a couple places.
Cook a few degress more,if it needs it.
Pull from heat and let set well wrapped in a well insulated cooler for a couple hrs.
It should rise another 5�and continue to slow cook.
Set on counter,with foil loosened for about 1/2 hr and slice pencil thick across the grain.
This should get you close enough to make adjustments for the next time.
Our choice flats seem to need a higher internal temp than our choice packers.
You can drain the liquid into a measuring cup and refrigerate to get the fat off the top.
It will be flavorful,but probably salty.
You can cut it with a little losalt stock or water and a couple Tbsp of sauce.
It goes pretty good as a thin gravy ,over sliced brisket.
Hope this helps a little.